The present invention relates in a first aspect to a mounting device for reducing short-circuiting forces that are transmitted from a stator core to a stator body in a rotating electric machine.
In a second aspect the present invention relates to a rotating electric machine incorporating mounting devices of the above-mentioned type.
The invention is applicable to rotating electric machines such as synchronous machines and normal asynchronous machines. The invention is also applicable to other electric machines such as dual-fed machines, and to applications in asynchronous static current converter cascades, outerpole machines and synchronous flow machines provided their windings are made up of insulated electric conductors, preferably operating at high voltages. By high voltages is meant in the first places electric voltages in excess of 10 kV. A typical working range for the device according to the invention may be of 36 kV-800 kV.
The invention is in the first place intended for use with a high-voltage cable of the type built up of an electric conductor composed of a number of strand parts, a first semiconducting layer surrounding the electric conductor, an insulating layer surrounding the first semiconducting layer, and a second semiconducting layer surrounding the insulating layer, and its advantages are particularly prominent here. The invention refers particularly to such a cable having a diameter within the interval 20-200 mm and a conducting area within the interval 80-3000 mm2.
Such applications of the invention thus constitute preferred embodiments thereof.
Similar machines have conventionally been designed for voltages in the range 15-30 kV, and 30 kV has normally been considered to be an upper limit. This generally means that a generator must be connected to the power network via a transformer which steps up the voltage to the level of the power network, i.e. in the range of approximately 130-400 kV.
A conductor is known through U.S. Pat. No. 5,036,165, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glassfiber. It is also known to provide conductors in a dynamo-electric machine with such an insulation, as described in U.S. Pat. No. 5,066,881 for instance, where a semiconducting pyrolized glassfiber layer is in contact with the two parallel rods forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glassfiber. The pyrolized glassfiber material is described as suitable since it retains its resistivity even after the impregnation treatment.
In rotating electric machines the stator core is attached to the stator body by mounting devices.
Conventional mounting devices consist of a guide bar, a beam and a mounting bolt. The guide bar is used to guide the stator lamination segments when laying the plates for the laminated core. The beam is welded into the stator body. The mounting bolt secures the guide bar to the beam and is arranged with the bolt head recessed in the guide bar and attached in the beam by a screw joint. (See FIG. 3.) The mounting bolt is thus shorter than the thickness of the beam. The package with guide bar, bolt and beam is repeated a number of times in peripheral direction of the stator. Since this connection between laminated core and stator body is relatively rigid, forces are transmitted from the stator core to the stator body and the base in the event of a short circuit. Transient short-circuiting forces are thus transmitted directly into the base. Furthermore, the manufacturing procedure for conventional mounting devices is relatively complicated and expensive. A specially-manufactured bolt is used, for instance.
The object of the present invention is to solve the problems mentioned above. This is achieved with a mounting device for reducing short-circuiting forces that are transmitted from a stator core to a stator body in a rotating electric machine as described herein, and a rotating electric machine comprising mounting devices of the above type. The rotating electric machine comprises a stator. The stator core is composed of a number of packs, each of which includes a number of metal sheets, or of a number of metal sheets, each pack or metal sheet having two identical grooves arranged for cooperation with wedge members designed to joint together packs or metal sheets. The stator body comprises beams, each connected to a wedge member. The mounting device according to the present invention is characterized in that windings are drawn through slots in the stator, wherein the windings consist of high-voltage cable and that the mounting device comprises a connector arranged through a through-hole in the beam and secured in the wedge member in order to connect the beam and wedge member, wherein the cross-sectional area of said hole at right angles to its longitudinal axis being greater than a cross-sectional area of the connector at right angles to the longitudinal axis of the connector, so as to permit sliding between the wedge member and the beam in the event of short-circuiting.
The mounting device according to the invention greatly reduces the forces transmitted from the stator core to the stator body in the event of short circuits. The mounting device is easy and quick to produce, as well as being relatively inexpensive.
In machines according to the invention the windings are preferably of a type corresponding to cables with solid, extruded insulation, such as those now used for power distribution, e.g. XLPE-cables or cables with EPR-insulation. Such a cable comprises an inner conductor composed of one or more strand parts, an inner semiconducting layer surrounding the conductor, a solid insulating layer surrounding this and an outer semiconducting layer surrounding the insulating layer. Such cables are flexible, which is an important property in this context since the technology for the device according to the invention is based primarily on winding systems in which the winding is formed from cable which is bent during assembly. The flexibility of a XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable 30 mm in diameter, and a radius of curvature of approximately 65 cm for a cable 80 mm in diameter. In the present application the term xe2x80x9cflexiblexe2x80x9d is used to indicate that the winding is flexible down to a radius of curvature in the order of four times the cable diameter, preferably eight to twelve times the cable diameter.
The winding should be constructed to retain its properties even when it is bent and when it is subjected to thermal stress during operation. It is vital that the layers retain their adhesion to each other in this context. The material properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. In a XLPE-cable, for instance, the insulating layer consists of cross-linked, low-density polyethylene, and the semiconducting layers consist of polyethylene with soot and metal particles mixed in. Changes in volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers in relation to the elasticity of these materials, the radial expansion can take place without the adhesion between the layers being lost.
The material combinations stated above should be considered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range of 10xe2x88x921-106 ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the scope of the invention.
The insulating layer may consist, for example, of a solid thermoplastic material such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polymethyl pentene (PMP), cross-linked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.
The inner and outer semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in.
The mechanical properties of these materials, particularly their coefficients of thermal expansion, are affected relatively little by whether soot or metal powder is mixed in or notxe2x80x94at least in the proportions required to achieve the conductivity necessary according to the invention. The insulating layer and the semiconducting layers thus have substantially the same coefficients of thermal expansion.
Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl graft polyethylene, ethylene-butyl-acrylate-copolymers and ethylene-ethyl-acrylate copolymers may also constitute suitable polymers for the semiconducting layers.
Even when different types of material are used as base in the various layers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with combination of the materials listed above.
The materials listed above have relatively good elasticity, with an E-modulus of E less than 500 MPa, preferably  less than 200 MPa.
The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbed in the radial direction of the elasticity so that no cracks appear, or any other damage, and so that the layers are not released from each other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as the weakest of the materials.
The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconducting layer is sufficiently large to enclose the electrical field in the cable, but sufficiently small not to give rise to significant losses due to currents induced in the longitudinal direction of the layer.
Thus, each of the two semiconducting layers essentially constitutes one equipotential surface and the winding, with these layers, will substantially enclose the electrical field within it.
There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulating layer.
The above mentioned and other advantageous embodiments of the present invention are stated in the dependent Claims.